Surveys of what people eat are based on only two days of the year

Researchers trying to understand the obesity epidemic frequently ask that question. So do public-health scientists seeking links between food and disease. Nutritionists want to know how many people are getting enough vitamins.

But answering the question is complicated. The simplest way is to ask people what they ate yesterday, do it again a few days, weeks or months later, take an average of the two...